VAT can be claimed back for all none EU residents. All VAT is refunded at airport. Typically if your friends buys at LV Paris, she/he will be given paperwork at the store when he/she shows his/her Passport. With that Passport, he/she can take the paperwork to get it stamped when he/she departs from an EU airport to a none EU destination. Make sure that LV item is in his/ her carry on to show stamping official if requested. With the stamped paperwork, your friend can go to a place (like Global Blue) inside departing area to get a refund. He/ she would want to get the refund through the credit card that he/she used to purchase the LV item.
You can get the estimated refund from here:
http://www.globalblue.com/tax-free-shopping/refund-calculator/
He/she can also purchase duty free shops inside departure zone in an airport. In that case, there is no need for paperwork. Just show passport and you get de-taxed prices. There is an LV at Heathrow terminal 5. I think there may be one at terminal 3 although terminal 5 is the bigger store and presumably has more stuff. The de-tax price is 17% off UK online price. I believe that may be the only place that has a proper LV store inside airport. I’m not aware of a LV store at airport in Paris.
Even though Paris VAT is around 20%, after paperwork cost, you will only get around 12% back. So to see where you can get the best deal, it’ll come down to currency exchange and amount of money you actually get back. You can do the calculation from the link above to get a good idea about where is the best place for you to get your stuff.
Also I believe you will get the most VAT back from countries that have the highest VAT rate. For example Italy’s VAT is 22% and France’s VAT is 20%. So in theory, you would get a little bit more VAT back when you shop in Italy since places like Global Blue take the same amount of cut for administrative work whether you buy in Italy or France.